We plan to determine whether CO2 plays a role in coronary flow autoregulation, and to then determine the extent that it participates in this mechanism. Measurements will be made of myocardial Pco2, coronary sinus Pco2 and Po2, arterial Pco2 while left main coronary flow is fixed and myocardial O2 consumption is altered. Coronary vascular resistance will be continuously measured.